The slinky is wiggled 10 times in 5 seconds. That's 2 times PER second, or 2  with units 1/sec.

That's

f=2 [1/sec].

Given that the distance between wavecrests is

l = 0.5 [meters]

We write

v = f l

and with numbers:

v= 2 [1/sec] x 0.5 [m] = 1 [m/sec]